1. Technical Field
The present invention relates to a polarization-multiplexed signal receiver, a polarization multiplexing system and a polarization-multiplexed signal receiving method using polarization-multiplexed signal, and, more particularly, to a polarization-multiplexed signal receiver, a polarization multiplexing system and a polarization-multiplexed signal receiving method which compensate for imperfection of receiving condition.
2. Background Art
In order to realize a superspeed optical transmission system beyond 40 Gb/s, a polarization multiplexing technology is being adopted. In the polarization multiplexing technology, two pieces of independent signal information are transmitted by utilizing two polarization states on an identical wavelength orthogonal to each other. By adopting the polarization multiplexing technology, a transmission bit rate can be made double while maintaining a symbol rate, that is, while maintaining a modulation frequency. A technology about a transmission system using the polarization multiplexing technology is disclosed in patent document 1 (Japanese Patent Application Laid-Open No. 2010-251851), for example.
However, when polarization-multiplexed signal carrying two pieces of independent signal information in each of two orthogonal polarized waves is received and the two pieces of signal information are taken out from the polarization-multiplexed signal, a quality of the signal information declines due to degrading of orthogonal nature of the two polarized waves.
Accordingly, in patent document 2 (Japanese Patent Application Laid-Open No. 2010-178091), there is disclosed polarization-multiplexed signal receiver which includes a polarization controller which controls a polarization state of received polarization-multiplexed signal in the foremost stage and performs feedback control of the polarization controller so that a total bit error ratio of X polarization channel signal and Y polarization channel signal made by applying polarization separation may become the smallest.